Laser printers typically include a printhead for generating a scanning laser beam to selectively charge an image that is to be printed onto a surface of a photosensitive drum from which the image will subsequently be transferred to a medium that is to be printed. Typically, the printhead will comprise a laser/diode pre-scan assembly for generating a laser beam, a scanner assembly to sweep the beam in a scan direction and a post-scan assembly to focus the beam and direct it onto the surface of the photosensitive drum at a proper position.
A printhead for a modern color laser printer may include four separate laser diodes to generate four separate laser beams corresponding, for example, to the colors yellow, cyan, magenta and black. In the pre-scan optical assembly, the individual beams are collimated and directed onto facets of a single rotating polygonal mirror in the scanning assembly. The facets of the rotating mirror sweep the individual beams across the surfaces of a plurality of mirrors and through f-theta lenses within the post-scan assembly. The beams then scan across surfaces of four corresponding photosensitive drums within the printer.
Referring now to FIG. 1, a conventional laser diode/pre-scan optical arrangement for use in a laser printer is shown diagrammatically and referred to generally by reference numeral 100. The conventional arrangement 100 includes a first laser diode 110 emitting a first laser beam 112, having a first beam center axis 114.
The first laser beam 112 diverges in both a process direction P and in a scan direction S upon leaving the first laser diode 110. In FIG. 1, the scan direction S is a direction in and out of the plane of the paper and is indicated by a point S. In the illustrated conventional arrangement 100, the first laser beam 112 diverges in the process direction P at an angle of about 8 degrees and in the scan direction S at an angle within a range of about 25 degrees to about 35 degrees.
A first structure 116, defining a first aperture 118, is positioned in the path of the first laser beam 112 such that a center portion 120 of the first laser beam 112 passes through the first aperture 118 and a peripheral portion of the first laser beam 112 represented by rays 122 and 124 is blocked by the first structure 116. The first aperture 118 is generally oval in shape.
After passing through the first aperture 118, the center portion 120 of the first laser beam 112 strikes a first surface 126 of a first collimation lens 128.
The first collimation lens 128 has optical power in the process direction P and in the scan direction S. The first collimation lens 128 further has an optical axis 130 passing through a mechanical center 132 of the first collimation lens 128. In the conventional arrangement 100 illustrated in FIG. 1, the first collimation lens 128 is positioned such that the optical axis 130 is substantially coaxial with the first beam center axis 114.
The first collimation lens 128 has a focal length in the focus direction F defined as a distance between the mechanical center 132 of the lens 128 and a point (not shown) where light rays that are parallel with the lens 128 optical axis 130 will converge to a point after passing through lens 128. In the conventional arrangement illustrated in FIG. 1, the first collimation lens 128 is positioned relative to the first laser diode 110 in the focus direction F such that a distance F1 between the point where the first laser beam 112 is emitted from the first laser diode 110 and the mechanical center 132 of the first collimation lens 128 is substantially equal to the focal length of the first collimation lens 128. In this fashion, the rays of the generally diverging first laser beam 112 emitted from the first laser diode 110 are collimated by the first collimation lens 128 such that a substantially collimated first laser beam 134 comprising substantially parallel rays 136, 138 and 140 is created as the first laser beam 114 passes through the first collimation lens 128.
The substantially collimated first laser beam 134 now strikes a pre-scan lens 142. The pre-scan lens 142 is a cylindrical lens having optical power in the process direction P only. The pre-scan lens 142 causes the rays 136, 138 and 140 of the substantially collimated first laser beam 134 to bend inward in the process direction P and further causes the rays 136, 138 and 140 to converge in the process direction P such that a converging first laser beam 144 comprising rays 146, 148 and 150 is created. The beam 144 is directed toward and converges to a point 152 in the process direction on a surface 154 of a scanner mirror, shown only partially in FIG. 1.
The conventional arrangement 100 also includes a second laser diode 158 emitting a second laser beam 160, having a second beam center axis 162. In the conventional arrangement 100 illustrated, the second laser diode 158 is separated from the first laser diode 110 in the process direction P such that the second beam center axis 162 is separated from the first beam center axis 114 by a distance P1 in the process direction P.
The second laser beam 160 diverges in both the process direction P and in the scan direction S upon leaving the second laser diode 160. The second laser beam 160 diverges in the process direction P at an angle of about 8 degrees and in the scan direction S at an angle within a range of about 25 degrees to about 35 degrees.
A second structure 164 defining a second aperture 166 is positioned in the path of the second laser beam 160 such that a center portion 168 of the second laser beam 160 passes through the second aperture 166 and a peripheral portion of the second laser beam 160 represented by rays 170 and 174 is blocked by the second structure 164. The second aperture 166 is generally oval in shape.
After passing through the second aperture 166, the center portion 168 of the second laser beam 160 strikes a first surface 176 of a second collimation lens 178. In the illustrated conventional arrangement 100, the structure 164, defining the aperture 166, is positioned about 1 mm in the focus direction F from the first surface 176 of the second collimation lens 178.
The second collimation lens 178 has optical power in the process direction P and in the scan direction S. The second collimation lens 178 further has an optical axis 180 passing through a mechanical center 182 of the second collimation lens 178. In the conventional optical arrangement illustrated in FIG. 1, the second collimation lens 178 is positioned such that the optical axis 182 is substantially coaxial with the second beam center axis 162.
The second collimation lens 178 has a focal length in the focus direction F defined as a distance between the mechanical center 182 of the lens 178 and a point (not shown) where light rays that are parallel with the optical axis 180 of the lens 178 will converge to a point after passing through the lens 178. In the conventional arrangement 100 illustrated in FIG. 1, the second collimation lens 178 is positioned relative to the second laser diode 158 in a focus direction F such that a distance F1 between the point where the second laser beam 160 is emitted from the second laser diode 158 and the mechanical center 182 of the second collimation lens 178 is substantially equal to the focal length of the second collimation lens 178. In this fashion, the rays of the generally diverging second laser beam 160 emitted from the second laser diode 158 are collimated by the second collimation lens 178 such that a substantially collimated second laser beam 184 comprising substantially parallel rays 186, 188 and 190 is created as the second laser beam 160 passes through the second collimation lens 178.
The substantially collimated second laser beam 184 now strikes the pre-scan lens 142. The pre-scan lens 142 is a cylindrical lens having optical power in the process direction P only. The pre-scan lens 142 causes the rays 186, 188 and 190 of the substantially collimated second laser beam 184 to bend inward in the process direction P and further causes the rays 186, 188 and 190 to converge in the process direction P such that a converging second laser beam 192 comprising rays 194, 196 and 198 is created. The second beam 192 is directed toward and converges to a point in the process direction that is near or at the same point 152 on the surface 154 of the scanner mirror 156 where the converging first laser beam 144 strikes the surface 154 of the scanner mirror 156.
In the conventional optical arrangement illustrated, the distance P1 in the process direction between the first beam center axis 114 and the second beam center axis 162 is substantially equal to a distance P2 between the optical axis 130 of the first collimation lens 128 and the optical axis 180 of the second collimation lens 178. Additionally, as previously mentioned, the first and second beam center axes 114 and 162 of the first and second laser beams 112 and 160 are substantially coaxial with the first and second optical axes 130 and 180, respectively, of the first and second collimation lenses 128 and 178. As a result, the first and second collimation lenses 128 and 182 serve to collimate the first and second laser beams 112 and 160, respectively, creating the substantially collimated laser beams 134 and 184.